Component: Soakaways

Description

Soakaways are square or circular excavations either filled with rubble or lined with brickwork, pre-cast concrete or polyethylene rings/perforated storage structures surrounded by granular backfill. They can be grouped and linked together to drain large areas including highways. The supporting structure and backfill can be substituted by modular or geocellular units.

Not suitable for locations where infiltration water may put structural foundations at risk, or where infiltrating water may adversely affect existing drainage patterns

Not appropriate for draining polluted runoff

Increased risk of groundwater pollution

Some uncertainty over long-term performance and possible reduced performance during long wet periods

Where the property owner is responsible for operation and maintenance, performance difficult to guarantee.

Where component can be used

Residential: Yes

Commercial/industrial: Yes

High density: Yes

Retrofit: Yes

Contaminated sites: No

Sites above vulnerable groundwater: No

Performance

Peak flow reduction: Good

Volume reduction: Good

Water quality treatment: Good

Amenity potential: Poor

Ecology potential: Poor

Quantity

Infiltration techniques:

provide storage for runoff in an underground chamber, lined with a porous membrane and filled with coarse crushed rock.

enhance the natural ability of the soil to drain the water. They do this by providing a large surface area in contact with the surrounding soil, through which the water can pass.

The amount of water that can be disposed of by a soakaway within a specified time depends mainly on the infiltration potential of the surrounding soil. The size of the device and the bulk density of any fill material will govern storage capacity.

Quality

Runoff is treated in different ways by a soakaway. These include:

physical filtration to remove solids

adsorption onto the material

biochemical reactions involving micro-organisms growing on the fill or in the soil.

The level of treatment depends on the size of the media and the length of the flow path through the system, which controls the time it takes the runoff to pass into the surrounding soil. Pre-treatment may be required before polluted runoff is allowed into a soakaway.

Amenity

Soakaways are easy to integrate into a site, but they offer very little in the way of amenity or biodiversity value as they should be completely underground and water should not appear on the surface.

They do, however, increase soil moisture content and help to recharge groundwater, thereby helping to mitigate problems of low river flows.